Drop on demand inkjet technology for producing printed media has been employed in commercial products such as printers, plotters, and facsimile machines. Generally, an inkjet image is formed by selectively ejecting ink drops from a plurality of drop generators or inkjets, which are arranged in a printhead or a printhead assembly, onto an image substrate. For example, the printhead assembly and the image substrate are moved relative to one other and the inkjets are controlled to emit ink drops at appropriate times. The timing of the inkjet activation is performed by a printhead controller, which generates firing signals that activate the inkjets to eject ink. The image substrate may be an intermediate image member, such as a print drum or belt, from which the ink image is later transferred to a print medium, such as paper. The image substrate may also be a moving web of print medium or sheets of a print medium onto which the ink drops are directly ejected. The ink ejected from the inkjets may be liquid ink, such as aqueous, solvent, oil based, UV curable ink or the like, which is stored in containers installed in the printer. Alternatively, the ink may be loaded in a solid form that is delivered to a melting device, which heats the solid ink to its melting temperature to generate liquid ink that is supplied to a print head.
Variations in inkjets may be introduced during print head manufacture and assembly. The variations include differences in physical characteristics, such as inkjet nozzle diameters, channel widths, or lengths, or differences in electrical characteristics, such as thermal or mechanical activation power for the inkjets. These variations may result in different volumes of ink being ejected from the inkjets in response to the same magnitude or same frequency firing signal. To compensate for these differences some previously known printers perform a process to normalize the firing signal for each inkjet within a printhead. Thus, normalizing the electrical firing signals that are used to activate individual inkjets enable all of the inkjets in a printhead to generate ink drops having substantially the same drop mass.
Another issue that arises during operation of an inkjet printer is intermittent, weak, or missing inkjets. Specifically, some inkjets fail either completely or partially so they no longer perform as expected to eject ink onto an image substrate. A method for compensating for such inkjets is disclosed in U.S. Pat. No. 7,021,739 to Burke et al. and which is assigned to the assignee of the present application. The method disclosed in that patent disables the inoperative inkjet and uses surrounding inkjets to compensate for the missing, intermittent, or weak inkjet. The printing to be done by the disabled inkjet is performed by one or more of the surrounding inkjets on one or more additional image substrate passes. Thus, this approach slows the printing process because additional substrate passes are required.